<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>Document</title>
	<meta name="viewport" content="width=device-width, initial-scale=1.0">
	<style>
		*{margin:0;padding:0;}
		@media screen and (min-width:640px){
			html{font-size:100px;}
		}
		@media screen and (max-width:640px){
			html{font-size:80px;}
		}
		

		header{
			height:.5rem;
			background-color:#f99;
			color:#fff;
			text-align: center;
			line-height:.5rem;
			font-size: .2rem;
		}
	</style>
</head>
<body>
	<header>头部</header>
</body>
</html>